The invention relates to a process for producing opaque quartz glass by providing a mixture comprising SiO2 particles and an additive that is volatile at a melting temperature, forming of a preform from the mixture and vitrification of the preform by heating at a melting temperature, with an advancing melt front in the preform. Furthermore, the invention concerns an article of opaque porous quartz glass.
In addition, the invention concerns an article of opaque porous quartz glass with an opening enclosed by an inner wall.
Opaque quartz glass is used to manufacture preforms for thermal applications where good heat insulation and high temperature stability are important. Increasing demands are made regarding the purity of such quartz glass preforms. Application examples may include uses in the semiconductor industry where opaque quartz glass is employed for tubes, bells and flanges for diffusion tubes. Opacity is present in low-purity quartz glass due to impurities contained therein. By contrast, with pure quartz glass source materials, opacity of the preform is achieved by pores in the quartz glass. In this context opacity means low transmission (less than a percent) both in the visible (between approx. 350 nm and 800 nm) and the IR spectra (approx. from 750 nm to 4,800 nm). The subject of this invention is the production of opaque quartz glass from pure source materials.
A process of this kind for the production of opaque quartz glass from pure source materials is described in EP A1 816,297. It is proposed there to mix high purity amorphous SiO2 particles of purified natural crystalline quartz granulate or of synthetically produced amorphous SiO2 with pure powdered silicon nitride (Si3N4), to place the powder mix in a graphite mold lined with graphite felt and to heat it in an electric furnace at between 1,400xc2x0 C. and 1,900xc2x0 C., either in a vacuum or in an inert gas atmosphere.
The heating temperature and duration is chosen such that the SiO2 particles are completely melted, forming the article. The lower temperature limit of 1,400xc2x0 C. mentioned there is given by the melting temperature of the SiO2 particles used, while melting at a temperature above 1,900xc2x0 C. results in very large bubbles which reduce the mechanical strength of the quartz glass. During the melting, the boundary of the softening and melting quartz glass progresses as the xe2x80x98melting frontxe2x80x99 from the graphite mold radially to the interior. At the same time gaseous components such as nitrogen are released due to thermal decomposition of the Si3N4 powder. The gaseous components form bubbles in the softened quartz glass, producing the desired opacity of the preform.
A preform manufactured according to the known process is composed of opaque glass with a specific density between 1.7 and 2.1 g/cm3, and contains between 3xc3x97105 and 5xc3x97106 bubbles/cm3 of closed bubbles with a diameter between 10 and 100 mm, with a total bubble surface between 10 and 40 cm2/cm3 and a homogenous bubble distribution.
Devitrification of the opaque quartz glass results in brittleness and reduced resistance to temperature change. In order to avoid this the known process proposes use of high purity source materials. However, contamination of the preform takes place during the manufacturing process as well. In the known process, sources of contamination can be the mold, the graphite felt and the melting atmosphere. In addition, remnants of additives that were not completely, or not at all transformed, can affect the quality of the preform.
The object of the invention is to provide a process for the manufacture of pure opaque quartz glass where the risk of contamination during the manufacturing process is reduced, and an article of pure opaque quartz glass distinguished by high resistance to temperature change, strength and chemical durability.
As concerns the process, the object is achieved on the basis of the process described initially, in that a preform is formed having an inner bore and that the heating takes place in such a way that the melt front advances from the inner bore to the outside.
The preform is formed either from loose fill or from mechanically, chemically or thermally pre-compacted porous mixture of amorphous or crystalline SiO2 particles and an additive. The additive is generally present as a powder or a liquid.
The melt front is an inexact boundary region between melted and partially melted material. Open pores and channels are present in the partially melted material, while closed pores are present in the melted material.
xe2x80x9cMelting temperaturexe2x80x9d means the highest temperature measured during the melting at the wall of the inner bore of the preform.
The preform is heated from the inner bore so that the melt front advances from the inner bore through the wall of the preform to the outside. Shape and location of the inner bore are not decisive for the invention; in the simplest case the inner bore is a central through bore.
According to the invention the melt front advances from the inner wall of the inner bore through the preform to the outside. Sublimable impurities pass into the gas phase. The relevant impurities here are primarily those escaping from the SiO2 particles or from the additive at the melting temperature, or originating in the heat source or the heating atmosphere. The impurities are driven by the melt front to the outside in the direction of the preform regions which are still porous.
The additive is volatile at the melting temperature and releases gases at the melting temperature. The gases are created by transformation (evaporation or decomposition) of the additive and leadxe2x80x94as is intendedxe2x80x94to the formation of bubbles in the region of the softened quartz glass. As the melt front advances to the outside the gases reach the boundary region of the preform where they can escape or be suctioned off. Remnants of the additive can impair the devitrification resistance of the opaque quartz glass; however, since the central regions around the inner bore are exposed to the melting temperature the longest, the transformation of the additive there is complete or most advanced. Since the gas being created there is being driven to the outside, the porosity of the melted preform in this region is especially low and increases toward the outside. In this the process according to the invention differs from the known process described initially. In the latter, the melt front advances radially from the outside to the inside so that the contaminantsxe2x80x94originating for example from the graphite mold or the graphite feltxe2x80x94preferably accumulate in the central region of the preform where they have in general the most. damaging effect, and cannot be removed.
Due to the effect of the melt front advancing from the inside to the outside as described above in more detail, the danger of contamination of the opaque quartz glass is reduced in the manufacturing process according to the invention. The heat source in the inner bore of the preform is surrounded by like material (SiO2 particles) acting simultaneously as the outer thermal insulation of the heat source. Contamination by foreign insulating material is therefore prevented.
It is not necessary, and in view of maintaining as high a purity of the melted preform as possible also generally not desired that the melt front should advance through the entire wall thickness of the preform. A remaining layer of unmelted SiO2 particles facilitates removal of the melted preform from the mold, contributes to the removal of gases during the melting and prevents diffusion of contaminants into the preform from the outside, for example from foreign mold material.
Advantageously, the preform is heated by regions (zones) along a longitudinal axis of the inner bore. This variant of the process permits a particularly high melting temperature. A high melting temperature (for example above 1,900xc2x0 C.) leads to a complete transformation of the chosen additive. Remnants of the additive in the melted preform are thus avoided. Also, in this variant of the process, additives such as Si3N4 can be used without problems, as otherwise, at high temperatures such additives tend to produce undesirably large bubbles at the expense of smaller bubbles. This is achieved by heating the preform successively and by region along the inner bore. The heat source is moved continuously, or in small steps relative to the inner wall, along the inner bore (the preform and/or heat source can be moved in a kinetically equivalent setup). The heat source creates in the preform a heated and softened region of low viscosity, hereinafter called the softening zone. The softening zone moves together with the heat source along the inner bore. Due to this constant shifting of the heating zone every region of the preform is exposed to the melting temperature only for a short time so that the softening zone also forms just for a correspondingly brief time period and then cools down immediately. The bubbles created in the softening zone are immediately xe2x80x9cfrozenxe2x80x9d during the cooling. This prevents a growth of bubbles due to low viscosity over a longer time period. Melting of the preform may require multiple back and forth movements of the heat source along the longitudinal axis of the inner bore; however, the melting is preferably accomplished in one pass.
It has been shown to be favorable to rotate the preform along the longitudinal axis of the inner bore. The rotating assures an even heating of the preform and temperature spikes are avoided.
An especially favorable variant has been one where an electric arc was used to heat the preform, the melting temperature having been set to above 1,900xc2x0 C. The electric arc discharges inside the inner bore of the preform. The mixture of SiO2 particles and an additive is exposed to particularly high temperatures above 1,900xc2x0 C. when heated by an electric arc. Due to the high temperatures, diffusion and other material exchange processes are accelerated. Contaminants, especially gaseous ones can be effectively removed because they expand and escape to the outside before the melt front. The additive is transformed as completely as possible so that remnants of the additive are avoided. In order to reduce the risk of contamination of the preform even more, electrodes of highly pure materials are available for the electric arc. There is no contact between the electrodes and the wall of the inner bore. When the required purity is particularly high the inner bore is flushed by a gas or the heating atmosphere is evacuated.
The source material used for the process according to the invention (SiO2 particles) is pure or purified granulate of naturally occurring quartz. When the required purity of the opaque quartz glass is particularly high the preferred source material is a granulate produced in a granulating process from synthetic SiO2. It has been shown to be particularly useful to compact porous granulate prior to use. Compacting is accomplished by complete or partial sintering of the porous granulate. The higher density of the granulate reduces accumulation or absorption of contaminants.
An additive is preferably used comprising one or preferably several of the following components: silicon carbide (SiC), aluminum nitride (AlN), silicon nitride (Si3N4), zirconia (ZrO2), zircon mineral (ZrSiO4), carbon (C), or a substance containing carbon.
Supplementally, but as a preferable alternative, an additive is used containing nano-scale SiO2 powder. Such nano-scale SiO2 powder occurs in known manufacturing processes for synthetic quartz glass, such as in flame hydrolysis or oxidation of inorganic silicon compounds or during sol-gel processing of organic silicon compounds. Such nano-scale SiO2 powders are distinguished by a particularly great surface area ranging up to several hundred m2/g (BET surface). Surprisingly, it has been shown that such SiO2 powder is suitable as an additive in the context of this invention since during heating it releases gases which result in the desired opacity. The particular advantage of using nano-scale SiO2 powder as an additive is that it is a material of the same kind as the opaque quartz glass, so that introduction of foreign material is avoided.
As far as concerns the article of porous quartz glass, the above-mentioned object is achieved in that the component comprises an opening enclosed by an inner wall and that the inner wall is provided with an inner SiO2 surface layer having a thickness between 30 and 500 mm and a density of at least 2.15 g/cm3.
The article may be for example a hollow cylinder, a container, a flange, a crucible or bell of quartz glass. The article may be designed rotationally symmetrical. The free surface of quartz glass facing the opening of the article shall be called xe2x80x9cSiO2 surface layerxe2x80x9d or xe2x80x9csurface layerxe2x80x9d here.
The surface layer is free of pores or is low on pores, having a density of at least 2.15 g/cm3. This density is close to that of transparent quartz glass. Accordingly, the mechanical or chemical properties of the surface layer correspond to those of dense transparent quartz glass. Among other things the surface layer, and therefore the opaque article according to the invention, are distinguished by high mechanical strength and hardness, low abrasiveness, and high chemical durability. The SiO2 surface layer is composed of the same material as the remaining article. This has an advantageous effect on the temperature change stability of the article.
The surface layer of the opaque quartz glass component is produced by forming a starting form from loose fill or from mechanically, thermally or chemically pre-compacted porous amorphous or crystalline SiO2 particles and an additive which is volatile at a melting temperature. The starting form has an inner bore and is heated to the melting point in such a way that a melt front advances from the inner bore to the outside. Further information may be found in the above explanation of the process according to the invention. The preform obtained in this process has an inner opening which was received the highest amount of heat around the inner bore. After melting, this region forms the dense surface called the xe2x80x9csurface layerxe2x80x9d while the outer regions are porous. A desired opaque quartz glass article may be obtained from the preform by treatment using known methods (flame working, cutting, sawing, drilling).
The article according to the invention is particularly suitable for making containers, muffles, heat shields or pipes where temperature-stable, chemically durable, or dense inner surface of like material is important. In the article according to the invention these functions are handled by the surface layer, and the opaque wall contributes to heat retaining capacity.
The thickness of the surface layer is limited to about 500 mm so that the heat and visible radiation insulating effect remains intact. The minimum thickness of the said surface layer, 30 mm, is necessary to assure the above-mentioned mechanical and chemical properties of the layer and therefore of the article.
A particularly advantageous thickness of the surface layer has been shown to be between 50 and 200 mm.